Method for cleaning molding compound tablets

ABSTRACT

A method for cleaning molding compound tablets while the molding compound tablets are transferred from a tablet container to a molding equipment for encapsulating a microelectronic device. The method includes the steps of loading the molding compound tablets from the tablet container to a tablet parts feeder; transferring the molding compound tablets from the tablet parts feeder to the molding equipment along the transfer line; blowing hot air downwardly on the molded compound tablets as they move along the transfer line; commencing melting of the molded compound tablet, and any contaminants on the molded compound, with the hot air, such that the contaminants are incorporated into a resulting soft tablet dough; and injecting the soft tablet dough into the molding equipment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method for cleaning moldingcompound tablets that are used for encapsulating semiconductor devicepackages, and an apparatus therefor. More particularly, the presentinvention relates to a method for removing dust and/or othercontaminants from molding compound tablets that are used forencapsulating semiconductor device packages and an apparatus therefor.

2. Description of the Related Arts

The integrated circuit manufacturing process basically comprises waferpreparation, wafer fabrication, electric test (die sort) and assemblyprocesses. In the wafer preparation process, a crystal of silicon isfirst ground to the required diameter, then a designated flat side isground on, and then individual wafers are sawed from the crystal. Eachraw wafer is polished to optical flatness and to the required thickness.

In the wafer fabrication process, the semiconductor device or integratedcircuit (IC) is created in and on the wafer surface, which requires thata number of sophisticated processes be performed in a sequential manner.Although the actual number of steps the wafer goes through will varydepending on the type and complexity of the circuit being built duringthe wafer fabrication process, three basic operations are generallyperformed on the wafer: layering, where thin layers of differentmaterials are grown on or added to the wafer surface; patterning, whereportions of the thin layers are selectively removed from the wafer; anddoping, where the resistivity and conductivity type of selected regionsin the wafer are changed or altered by the addition of dopants.

After the electrically active regions are created through the dopingsequences in the wafer surface, a metallization process is performedwhereby the regions or components on and in the wafer surface are wiredtogether. After this metallization process, the circuit or device isoperationally complete. However, since the circuit is vulnerable tocontamination and the thin fragile metallization layer is susceptible toscratching, the wafer is covered with a layer that acts as acontamination barrier and protects the metal layer.

After the wafer fabrication process, the wafer is separated intoindividual chips and mounted and connected to a chip package for theelectrical die sort test. The good die which pass the tests are attachedto a pad of a lead frame, and the chip is electrically connected toleads through wires.

Finally, the chip is encapsulated with a molding compound to give shapeto the package body and to provide protection for the electricalinterconnections and chip from exterior environmental stresses.

The molding operation is generally carried out using a molding press.Tablets of a molding compound are melted by increasing the temperatureof the press, and the hot melted molding compound is injected and thencooled to room temperature to provide a package body. The moldingcompound is the most widely used plastic packaging material forencapsulating various types of microelectronic devices for commercialapplications. Transfer molding is the process used almost exclusivelywith molding compounds.

A molding compound is a granulated powder that is normally used in theform of a tablet (preform). In a conventional transfer molding process,one molding time cycle takes between 60 seconds and a few minutes. Itconsumes one to several tablets, which are preheated to a temperature ofabout 66° C. with a radio frequency (RF) pre-heater until it achievesthe consistency of a soft dough. These soft tablets are then used toencapsulate up to several hundred microelectronic devices using theconventional transfer molding press at a temperature of about 150 to190° C. (300 to 375° F.) and a pressure of about 4.1 to 8.3 Mpa (0.600to 1.20 ksi).

The tablets are stored until they are required for the molding process.However, the tablets may be stored for a considerable period of time,and during such storage, dusts or motes may contaminate them. Thesecontaminants may in turn cause the formation of voids in the moldedpackage body. As such, when they are introduced into the molding die,they may cause a change in the speed of molding compound that is flowinginto the molding die, resulting in a non-homogeneous molded body.Further, the contaminants may accelerate the aging of the molding pressand cause erroneous operation of the sophisticated sensors of theautomated molding equipment.

The tablet contamination problems described above account for more than50% of the total tablet-related problems. Accordingly, cleaning of themolding compound tablets is very important for successful encapsulation.

To minimize possible contaminating particles on the tablets, varioustechniques such as air blowing, brushing, vacuum cleaning or vibrationsare employed. All of these methods, however, only detach or drop thedust or other contaminants from the tablets. Therefore, the dusts maystill fly or float in the air, and must later be removed by a vacuumcleaner after being collected. The flying dusts may contaminate the justcleaned wafer, especially when the vacuum system does not remove all theflying dusts.

FIG. 2 is a schematic diagram of conventional cleaning system forremoving dusts or other contaminants from the molding compound tablets.

With reference to FIG. 2, the conventional cleaning system comprises atablet container 14, e.g., a tablet hopper carrying molding compoundtablets 18, and parts feeder 10, to which the tablets 18 are loaded fromthe container 14. Vacuum sweeper 16 is provided above the container 14and the parts feeder 10. When a tablet 18 in the container 14 is loadedto the parts feeder 10, the vacuum sweeper 16 starts operating tosuction the dusts or contaminants from the tablet 18.

The cleaned tablet 18 is transferred to transfer line 12, which movesthe tablet 18 to molding equipment 20. The transfer line 12 may beprovided with a vibrator 13 to vibrate the tablet 18 so as to detachcontaminants from the tablet 18.

However, the dislodged dusts or contaminants are not necessarily removedfrom the tablet and they may still fly or float in the air around thetablet, and thus contaminate the tablet.

SUMMARY OF THE INVENTION

Thus, an object of the present invention is to remove dusts or any othercontaminants from the molding compound tablets prior to using thetablets for encapsulating semiconductor device packages.

The above object can be accomplished by, for example, a process forcleaning the molding compound tablets by using a hot air blowingpre-heater. According to the present invention, a means is used to blowhot air, for example, at about 50 to 60° C., to melt or soften the dustsand the molding compound tablet so that the melted contaminants adhereto the softened tablet.

According to one aspect of the present invention, there is provided amethod for cleaning molding compound tablets while the molding compoundtablets are transferred from a tablet container to a molding equipmentwhere a microelectronic device is encapsulated using the moldingcompound tablets, the method comprising steps of:

(a) loading the molding compound tablets from the tablet container to atablet parts feeder, the tablet parts feeder being connected to atransfer line;

(b) transferring the molding compound tablets from the tablet partsfeeder to the molding equipment along the transfer line;

(c) blowing hot air downwardly on the molded compound tablets as theymove along the transfer line;

(d) commencing melting of the molded compound tablet, and anycontaminants on the molded compound, with the hot air, such that thecontaminants are incorporated into a resulting soft tablet dough; and

(e) injecting the soft tablet dough into the molding equipment.

According to other aspect of the present invention, there is provided anapparatus for cleaning molding compound tablets while the moldingcompound tablets are transferred from a tablet container to a moldingequipment where a microelectronic device is encapsulated with themolding compound tablets, the apparatus comprising: a tablet partsfeeder to which the molding compound tablets are loaded from the tabletcontainer; a transfer line for transferring the molding compound tabletsfrom the tablet parts feeder to the molding equipment; and means forblowing hot air on the molding compound tablets as they move along thetransfer line, wherein the molded compound tablets and any contaminantson the molded compound tablets are melted with the hot air, such thatthe contaminants are incorporated into a resulting soft tablet dough,the means for blowing hot air being separated a predetermined distanceabove the transfer line.

BRIEF DESCRIPTION OF THE DRAWINGS

These and various other features and advantages of the present inventionwill be readily understood with reference to the following detaileddescription taken in conjunction with the accompanying drawings, whereinlike reference numerals designate like structural elements, and, inwhich:

FIG. 1 is a schematic diagram of a cleaning system for removing dustsand other contaminants from the molding compound tablet according to thepresent invention; and

FIG. 2 is a schematic diagram of a conventional cleaning system forremoving dusts and other contaminants from the molding compound tablets.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in greater detail withreference to accompanying drawings. For simplicity and ease ofdiscussion, throughout the description of the invention and the claims,the term "contaminants" is generally used to refer to foreign particles,for example, dusts or motes, or any other foreign particles causing thecontamination problems referred to in the above discussion.

According to one aspect of the present invention, there is provided anapparatus for cleaning a molding compound tablet during its transferfrom a tablet container to a piece of molding equipment where amicroelectronic device is encapsulated with the molding compound tablet.FIG. 1 is a schematic diagram of such a cleaning system for removingcontaminants from the molding compound tablets according to the presentinvention.

Referring to FIG. 1, it is shown that the cleaning system of the presentinvention has the same structure as that of the conventional one shownin FIG. 2, except that the present invention further incorporates a hotair blowing means 100 and an additional vacuum sweeper 16A. Theembodiment of FIG. 1 also comprises a tablet container 14 which containsthe molding compound tablets 18, and tablet parts feeder 10 to which themolding compound tablets 18 are loaded from the tablet container 14.Vacuum sweeper 16 is provided above the tablet container 14 and theparts feeder 10.

When a molding compound tablet 18 from the tablet container 14 is loadedto the parts feeder 10, the vacuum sweeper 16 starts operating tosuction the contaminants from the tablet 18. This preliminarily cleanedtablet 18 is transferred to transfer line 12, for transferring thetablet 18 to molding equipment 20. A means for blowing 100 hot air blowsair downwardly on the molded compound tablet 18 to begin to melt anycontaminants on the molding compound tablet 18 as it also begins to meltthe molding compound tablet 18. The hot air causes the molding compoundtablet 18 to melt or soften to the consistency of a soft dough. Themelted contaminants are thus incorporated into the soft dough.

As shown in FIG. 1, the means for blowing 100 is located above thetransfer line 12. Also, as shown in FIG. 1, an additional vacuum sweeper16A is incorporated in the present invention, and is located below thetransfer line 12. The transfer line 12 may be provided with a vibrator13 to facilitate a more efficient cleaning of the molding compoundtablet 18.

In the embodiment shown in FIG. 1, the means for blowing 100 comprises,for example, a heating coil 22 for generating heat, rotating blades 28located above the heating coil 22, a fixing plate 24, and a rotatingshaft 26 fixed to the plate 24, which rotates the blades 28. The meansfor blowing 100 blows hot air of a predetermined temperature thatensures a melting or softening of the contaminants and the preliminarilycleaned tablet so that the contaminants are incorporated into the softtablet dough and thus cannot fly or float in the air. In effect, therotating blades 28 blow air over the heating coil 22 to create the hotair flow.

Even if there were any remaining free contaminants floating in the air,they can be removed by the additional vacuum sweeper 16A, which islocated below the transfer line. In FIG. 1, the additional vacuumsweeper 16A is vertically aligned with the means for blowing 100,although it need not be. The vacuum sweeper 16A may also be operated bya power source different from that for the vacuum sweeper 16 locatedabove the tablet parts feeder 10, although both vacuum sweepers may beoperated by the same power source.

The method for cleaning the molding compound tablets 18 will bedescribed hereinafter. The method of the present invention is based onthe fact that the molding compound becomes a soft dough when atemperature of about 60° C. to 70° C. is applied and is completelymelted at a temperature of about 175° C. Once the molding compound ismolded at a temperature of about 175° C., it does not transform anyfurther, but, tends to break if a temperature greater than 175° C. isapplied. The contaminants on the molding compound tablet 18 cangenerally be incorporated in the soft dough by heating the tablet to atemperature of about 50° C. to about 60° C.

In the present invention, when a molding compound tablet 18 in thetablet container 14 is loaded to the tablet parts feeder 10, the vacuumsweeper 16 starts operating to initially suction the contaminants fromthe tablet 18. This preliminarily cleaned tablet 18 is then transferredto transfer line 12, which moves the tablet to the molding equipment 20.As described above, however, contaminants may still remain on the tablet18, even after the initial vacuum suctioning process.

After the tablet 18 is loaded onto the transfer line 12, the means forblowing 100 hot air, which is located a predetermined vertical distanceabove the transfer line 12, starts to blow hot air downwardly over thetablet to begin to melt or soften the tablet. The temperature of hot airis between about 50° C. and about 60° C. The particular temperature mayvary, however, depending on the type of the molding compound tablet 18utilized.

The transfer line 12 may be provided with a vibrator 13, which detachescontaminants from the molding compound tablet 18 on the transfer line12. The operation of the blowing means 100 can be controlled so as to bein synchronization with the vibration of the transfer line 12.

The operating time for the blowing means 100 is important, since toolong a time of operation may cause chemical reactions within the moldingcompound tablet 18, resulting in a quality deterioration of the tablet.Therefore, for example, the blowing means 100 can be constructed tostart operating when the transfer line 12 starts vibrating upon theloading of the tablet 18 thereon, and to stop operating when thetransfer line 12 stops vibrating after the tablet 18 passes through theblowing means 100. The cycle time is generally on the order of one toten minutes.

The contaminants and the molding compound tablet 18 are sufficientlymelted or softened by the hot air from the blowing means 100, and thecontaminants are incorporated into the softened tablet and thus cannotfly or float in the air.

Even if there remain any free contaminants floating in the air, they canbe removed by using the additional vacuum sweeper 16A, which is locatedbelow the transfer line 12 as described above. The sweeper 16A alsosuctions the hot air blown from the blowing means 100. According to thepresent invention, therefore, the contaminants on the molding compoundtablet can be effectively removed by pre-heating the tablet using a hotair blower.

Although preferred embodiments of the present invention have beendescribed in detail hereinabove, it should be clearly understood thatmany variations and/or modifications of the basic inventive conceptsherein taught which may appear to those skilled in the art will stillfall within the spirit and scope of the present invention as defined inthe appended claims and their equivalents.

What is claimed is:
 1. A method for cleaning molding compound tabletswhile said molding compound tablets are transferred from a tabletcontainer to a molding equipment where a microelectronic device isencapsulated using said molding compound tablets, said method comprisingsteps of:(a) loading said molding compound tablets from said tabletcontainer to a tablet parts feeder, said tablet parts feeder beingconnected to a transfer line; (b) transferring said molding compoundtablets from the tablet parts feeder to the molding equipment along thetransfer line; (c) blowing hot air downwardly on the molded compoundtablets as they move along the transfer line; (d) commencing melting ofthe molded compound tablets, and any contaminants on the molded compoundtablets, with the hot air, such that the contaminants are incorporatedinto a resulting soft tablet dough; and (e) injecting the soft tabletdough into the molding equipment.
 2. The method of cleaning as in claim1, which further comprising a step of removing any free contaminantsfrom around the molding compound tablets by using a vacuum sweeperlocated below the transfer line, at the same time as said blowing step.3. The method of cleaning as in claim 1, wherein the hot air from theblowing step has a temperature of about 50° C. to 60° C.
 4. The methodof cleaning as in claim 2, wherein the hot air from the blowing step hasa temperature of about 50° C. to 60° C.
 5. The method of cleaning as inclaim 2, further comprising a step of vibrating said transfer lineduring said transferring step.
 6. The method of cleaning as in claim 5,wherein said vibrating step and said blowing step are startedsimultaneously.
 7. The method of cleaning as in claim 6, wherein saidvibrating step and said blowing step are stopped simultaneously.